The rejection of inorganic electrolytes and organic substances soluble in water with a selective permeable membrane such as a nano filtration membrane and a reverse osmosis membrane decreases due to degradation of the macromolecular material by the effect of oxidative substances and reductive substances present in water and by other causes, and it becomes difficult that the required quality of the treated water is obtained. This decrease may take place gradually during a long period of the use or suddenly due to an accident. When this decrease takes place, it is required that the ability of the membrane be recovered from the degraded condition without separating the membrane from the module to which the membrane is disposed and, where possible, the recovery be achieved while the operation of treating the supplied water is continued.
To satisfy the above requirement, processes for maintaining the rejection exhibited with an unused permeable membrane for a long time and repair processes for recovering the rejection with a permeable membrane exhibiting a decreased rejection are developed. For example, as the process for regenerating a selective permeable membrane on which organic substances having anionic groups are deposited, a process in which the permeable membranes is treated with an aqueous solution of an amphoteric surfactant having a quaternary amino group or a cationic surfactant is proposed (Patent Reference 1). As the process for maintaining the properties of a reverse osmosis membrane exhibited in the unused condition for a long time and recovering the rejection of salts with the membrane decreased by the use, a process in which the reverse osmosis membrane is brought into contact with an agent for treating a membrane such as polyvinyl methyl ether, polyvinyl alcohol and polyacrylic acid in the condition of a great concentration and then with the agent continuously in the condition of a small concentration is proposed (Patent Reference 2). As the process for improving the rejecting ability and the maintenance of the ability of a semipermeable membrane, a treatment process in which an auxiliary polymer having acetyl group is added to a semipermeable membrane is proposed (Patent Reference 3). As the agent for treating a semipermeable membrane which is applied not only to a used semipermeable membrane but also to an unused semipermeable membrane and improves permeation of the solvent and separation of solutes, a treatment agent which comprises a vinyl-based polymer having an organic group having acetoxy group and terminal carboxyl group as the side chain is proposed (Patent Reference 3). However, these processes and agents have problems in that the increase in the rejection is possible only to a small degree, the decrease in the flux of permeation is great, and the increased rejection is not sufficiently maintained.
As for the process or the agent used for treating a cellulose acetate membrane, for example, a process in which a portion of defect is coated with a liquid substance having compatibility with the membrane and exhibiting the plasticizing effect, followed by smoothing the formed surface is proposed as the effective process for repairing and recovering a small defect found after the membrane is set at a module (Patent Reference 5). However, in accordance with this process, the material of the membrane which can be treated is limited, and complicated operations such as heating are required. As the agent for enhancing separation of solutes which shows a small decrease in the permeability of water and is durable, an agent comprising a vinyl polymer having an alkoxyl group, carboxyl group and an alkoxycarbonyl group in the side chains is proposed (Patent Reference 6).
A nano filtration membrane having adsorbed polyethyleneimine having a molecular weight of 70,000 is reported (Non-Patent Reference 1). Although the rejection of NaCl is about 15%, a rejection of MgCl2 of 90% or greater is obtained at a pH of 4. However, the rejection of MgCl2 is decreased by 3% after 15 hours, and the main cause for the decrease is considered to be the small molecular weight of polyethyleneimine. As the process for treating a reverse osmosis membrane which can maintain the effect of decreasing the concentration of solutes in water passed through the membrane for a long time and can separate boron which is not dissociated in a non-electrolytic organic substance or in a neutral range with a great rejection, a process in which a pressure-resistant vessel in an apparatus for the membrane separation is packed with a reverse osmosis element having a polyamide skin layer and, thereafter, an aqueous solution of free chlorine containing bromine is brought into contact with the element, is proposed (Patent Reference 7). In accordance with this process, bromine is introduced to the surface of the membrane, and the surface is chemically modified. Since chlorine and bromine are used in great concentrations, it is necessary that a great care be taken for the control of the reaction conditions and also for the safety. Therefore, it is difficult that the operation of increasing the rejection with a membrane is conducted at the place of the use.    [Patent Reference 1] Japanese Patent Application Laid Open No. Showa 57(1982)-119804 (page 1)    [Patent Reference 2] Japanese Patent Application Laid Open No. Showa 53(1978)-28083 (pages 1 and 2)    [Patent Reference 3] Japanese Patent Application Laid Open No. Showa 50(1975)-140378 (page 1)    [Patent Reference 4] Japanese Patent Application Laid Open No. Showa 55(1980)-114306 (pages 1 and 2)    [Patent Reference 5] Japanese Patent Application Laid Open No. Showa 56(1981)-67504 (pages 1 and 2)    [Patent Reference 6] Japanese Patent Application Laid Open No. Showa 55(1980)-11048 (pages 1 and 2)    [Patent Reference 7] Japanese Patent Application Laid Open No. 2003-88730 (page 2)    [Non-Patent Reference 1] Urairi et al., Journal of Membrane Science, 70(1992) 153-162.